1. Field of the Invention
The present invention finds principal application within the field of marine seismic exploration. More particularly, the invention is concerned with a method and means for accurately determining the position of a towed marine seismic streamer.
2. Prior Art
In marine seismic prospecting, an exploration vessel tows a seismic streamer having a plurality of pressure sensitive detectors, commonly referred to as hydrophones. A source of seismic energy, such as an air gun or an explosive charge, it used to propagate pressure waves through the water into the underlying sea floor. Part of the energy will be reflected by subfloor geological discontinuities and subsequently detected by the hydrophones as pressure variations in the surrounding water. The mechanical energy of these pressure variations is transformed into an electrical signal by the hydrophones and transmitted through the streamer to recording apparatus aboard the vessel. The collected data may then be interpreted by those skilled in the art to reveal information about the subsea geological formations.
For the signals to be meaningful, it is necessary to know the precise location of the individual hydrophones at the time the pressure waves are detected. As the vessel is continually moving and as the streamer may extend for thousands of feet behind the vessel, accurate location of hydrophones is difficult.
Various systems have been developed to provide accurate information as to the location of the vessel, however, it is rare for the streamer to trail directly along the path of the vessel. While the streamer is attached to the stern of the vessel, the bulk of the streamer is submerged below the water surface through the action of depth controllers along the length of the streamer. As a result, the cross-track current velocity at the streamer depth may differ from the cross-track current affecting the vessel, thereby causing the streamer to trail at an angle to the vessel's course. Other factors, which are not necessary to enumerate, may also create a variance in the path of the streamer when compared to the vessel track.
One method of estimating the location of the streamer disclosed in the prior art relies upon the addition of a tail buoy radar reflector located at the end of the streamer. On-board radar systems may then be used under optimal sea conditions to find the end of the streamer and the location of the individual hydrophones interpolated. Such systems are generally unreliable however, and render the required data suspect.
A second method taught by the art relies upon very sensitive and expensive apparatus to measure the yaw and pitch angles of the streamer end adjacent the vessel. These data, coupled with magnetic compass headings taken along the streamer and the known depth of the streamer, permits one to empirically calculate the hydrophone locations.
In normal operations, the vessel travels at a speed of approximately 3 meters per second and sets off original seismic propagations approximately every 10 seconds. The use of seismic propagations at a shorter interval is limited by the time required for the dissipation of all reflected seismic waves. In particular, the use of an air gun at intervals of less than 4 seconds will not permit sufficient dissipation of the sound waves and will result in data that is difficult or impossible to evaluate due to the reflected noise. Thus, the use of an air gun, in combination with the hydrophones for range estimation, presents problems and does not allow for a lack of redundancy in precisely locating the hydrophones.